This invention relates to assemblies of components of molded and crosslinked polymer materials, and more particularly polymer components bonded together in an assembly, and then the polymers crosslinked to provide desired properties.
Molding of components from polymer materials has been known by various applications such as injection molding, extruding, blow molding, forming, AIR-CORE® pressure forming of extrusions, and other molding and forming processes. Molded components have been assembled together to form assemblies by mechanical and thermal methods, chemical processes, ultrasonic and vibratory welding, and other bonding. In one application, such as disclosed in U.S. Pat. Nos. 5,895,695 and 6,287,501, one polymer component was injection overmolded onto a second polymer component. In another example disclosed in U.S. Pat. No. 7,850,898, one extruded polymer component was molded over the end of a second extruded polymer component.
In the past, such assemblies were configured to pass a desired performance test. In plumbing applications, for example, a tube and fitting assembly for connection to a faucet or other fixture was required to withstand an internal water pressure under elevated temperature conditions without leaking. In certain appliance applications, a molded tank assembly was required to withstand an internal water or air pressure test without leaking. Other assemblies were required to withstand other performance specifications including pull tests, drop tests, impact tests, creep resistance tests, odor and taste tests, life tests, and other various strength and performance tests as desired.
For some applications in the Prior Art, the polymers used in polymer assemblies were crosslinked to achieve the desired performance. In the example of U.S. Pat. No. 6,287,501, the tube and the overmolded component were crosslinked to desired amounts using chemical crosslinking, e.g., by a silane process wherein some crosslinking occurred during molding and was completed in a hot bath after molding. When polyethylene was used for the application, the chemical crosslinked material was called PEX-B when silane was used as the crosslinking agent. PEX-B enabled molders to crosslink the components of the assembly as discussed in U.S. Pat. No. 6,287,501. With chemical crosslinking such as PEX-B, at least a part of the crosslinking occurred during the extrusion and molding processes. This limited the application of chemical crosslinking when creating assemblies by processes of overmolding or other molding, thermal, or welding assembly processes by which components are molded together by fusion or melting between the materials of the components. As crosslinking progressed, the increasing crosslink percentage meant that there were fewer thermoplastic bonding sites remaining for melt fusion between one component and another component during overmolding or other assembly, which formed a bond between components that was less robust. The less-robust connections were not desirable in certain applications such as pressurized water fittings, where connection failure could enable water to free-flow from the assembly for extended periods.
Crosslinking a polymer improves or enhances various properties of the material, with the amount of change often increasing with increasing amount of crosslinking. For example, crosslinking increases the thermal deflection and softening temperatures of polymers. For certain polymers, crosslinking may increase abrasion and chemical resistance, lower elongation, increase tensile strength, decrease stress cracking, and improve toughness. Other properties may be improved or enhanced by crosslinking certain polymers. As such, crosslinking of less expensive polymers such as polyethylene has been practiced as a way to extend the life and performance of the polymer to what is required for certain applications and not normally attained by the same polymer in its uncrosslinked state, for example hot and cold water applications. According to standards for certain applications, polyethylene must be crosslinked to a minimum of 65% crosslinked to meet the required performance parameter, meaning that 65% of the polymer is crosslinked and the balance of the polymer remains thermoplastic. In the past, performance was obtained by crosslinking each component separately to accomplish 65% minimum crosslinking. Components were crosslinked separately because each component needed its own level of irradiation under an electron beam to achieve the desired crosslinking. While this provided the specified crosslink amount in every part, the crosslinked components had to be assembled with limited ability to form bonded and molded connections because the crosslinking inhibited bonding. In the Prior Art, as illustrated for example by U.S. Pat. Nos. 5,895,695 and 6,287,501, to create a bond between the components, the overmolded portion had to be made at the earliest time when the base underlying polymeric profile was the least crosslinked to provide more thermoplastic bonding for a material to material bond.
To promote complete bonding between molded components in the assembly, components of uncrosslinked materials have been molded and bonded together and then the assemblies crosslinked after molding using radiation, by passing the assemblies under an electron beam. When polyethylene was used for the application, the material crosslinked by radiation was called PEX-C. For PEX-C, overmolding of a thermoplastic component onto a second component had to be done prior to crosslinking the second component so that the second component retained its thermoplasticity for a material to material bond.
Crosslinking is a process in which carbon atoms of polymer chains are joined together to form a network structure. Crosslinking forms a covalent chemical bond between the polymer chains, which are typically carbon to carbon bonds or a chemical bridge linking two or more carbon atoms. During the crosslinking process, exposing the polymer to radiation, such as an electron beam, displaces hydrogen atoms from the polymer chains, resulting in the formation of a free radical where each hydrogen was removed. Free radicals are highly reactive molecular fragments having one or more unpaired electrons. The free radicals are unstable and typically will seek another free radical or will react with unsaturated compounds to form a stable bond. A cross-link forms when a free radical on one polymer chain bonds with a free radical on another polymer chain linking the two chains together. Two or more chains can join together where a free radical is generated. Alternatively, a molecular bridge connects between free radical sites on two or more polymer chains to form a cross-link. As irradiation progresses, more and more bonds are formed to create a cross-linked structure. When crosslinked by exposure to electron beam, such as PEX-C, the cross-links typically formed are carbon to carbon bonds between the polymer chains. In chemical crosslinking using silane, a silane molecule such as vinyl trimethoxysilane is grafted onto the polymer chain. The silane molecule is typically grafted by using peroxide to generate a free radical on the polymer chain, to which the silane attaches. Then, the silane/polyethylene copolymer is crosslinked by exposure to water with the aid of a catalyst. The water enables hydrolysis and subsequent condensation reactions form cross-links in which the silane molecule forms stable siloxane (Si—O—Si) molecular bridges between the polymer chains.
In many polymeric applications, antioxidants were provided to inhibit oxidation of the polymer during molding and other processing as well as in its desired application. However, the antioxidants in the polymer during irradiation reacted with free radicals needed for crosslinking resulting in a decrease in crosslink density. For this reason, conventional wisdom in the prior art was to limit the use of antioxidants in order to obtain the desired crosslink percentage.
When crosslinking by radiation of a polymeric assembly of two or more components that may have varying wall thicknesses, the assembly was passed under the electron beam oriented in a purposeful orientation. Alternatively, for certain applications, multiple assemblies were placed into a box in a random orientation. In either event, the amount of crosslinking of one component in the assembly was always dependent upon the amount of crosslinking of the other components in the assembly. To achieve a desired crosslinking percentage in a specified component of the assembly, the crosslinking percentage of other components in the assembly may have been higher or lower than the specified component dependent upon the radiation delivered to crosslink the specified component.
To further explain, an electron beam source emits a constant stream of electrons at a set power level. When the assemblies passed through an electron beam and the electron beam impinged upon the assembly, the material forming the assembly closest to the source of the beam received the highest amount of radiation, and hence the most crosslinking, and the material furthest from the source of the beam received the least amount of radiation, and hence the least crosslinking. On a pass under the beam, the assemblies received a diminishing amount of radiation through the material further from the beam as the beam passed through the thickness of the material, and thereby non-uniform crosslinking. To compensate for the effect of diminishing radiation, the assemblies had to be turned over to orient the surfaces previously on the bottom to face the top, and passed under the beam again so that portions previously receiving a diminished amount of radiation received additional exposure to the electron beam. In some applications, each assembly had to be passed under the beam multiple times to achieve a minimum crosslinking percentage. Typically, one of the parts of the assembly would crosslink at a different rate than the other. In certain applications, one component of the assembly would reach a desired crosslinking percentage before the other, providing different crosslink percentages in different components in the assembly. Alternatively, the assembly was further irradiated until all of the parts had a crosslinking percentage greater than a desired amount. Portions of the assembly having thicker wall sections would absorb radiation reducing the crosslinking of portions of the assembly further from the electron beam beneath the thick sections further providing variable crosslink percentages. In any event, when crosslinking by radiation, the amount of crosslinking of one component was dependent upon the amount of crosslinking of the other components in the assembly.
While the prior processes obtain required crosslinking amounts, additional passes under the electron beam to bring lower crosslinked components up to a desired level increase the cost of the assembly. There remains a need for a method of making a polymeric assembly crosslinked by radiation where the crosslinking percentage of each component is independently controlled so that each component is sufficiently crosslinked to meet required performance parameters, providing a polymeric assembly wherein each component has about the same crosslink percentage or each component has different crosslink percentages as desired.